1. Field of the Invention
The present invention relates to a method and apparatus for controlling the amount of intake air in an internal-combustion engine having a supercharger and also having an idle-speed control unit.
2. Description of the Prior Art
Generally, in order to control the rotational speed of an engine when the throttle valve is in an idle position, i.e., in order to bring the actual idle speed of the engine to a desired idle speed, an air-bypass passage connecting the upstream side of the throttle valve to the downstream side thereof is provided in an intake-air passage of the engine. In addition, a control valve is provided in the air-bypass passage so as to control the rate of air flowing therethrough. In a closed loop control, i.e., in a feedback control for controlling the actual idle speed, the opening of the control valve is controlled dependent upon the difference between the actual idle speed and the desired idle speed so that the former conforms with the latter.
Note that if the operating condition does not satisfy the feedback control condition, the opening of the control valve is set at a so-called learning value calculated during the feedback control operation. As a result, when the operating condition again satisfies the feedback condition, control of the idle speed smoothly returns to a feedback control state.
On the other hand, in an internal-combustion engine having a supercharger, since the intake air is forcibly pressed by the exhaust-gas energy and is then supplied to the combustion chamber of the engine, the pressure of the intake-air passage at the downstream of the throttle valve becomes higher than the atmospheric pressure when the load of the engine increases. As a result, in the air-bypass passage, the pressure at the downstream side of the control valve becomes higher than that at the upstream side thereof so that the balance of pressure is reversed. Therefore, when the opening of the control valve of the air-bypass passage is set at the above-mentioned learning value, the air of the intake-air passage at the downstream side of the throttle valve may leak out through the air-bypass passage due to the opening of the control valve. Further, in this case, since oil which leaks out of the bearings of the turbine of the supercharger passes through the control valve, this valve may be contaminated by the leaked oil.